Recently, multi-protocol label switching (MPLS) has been used, which allows for the operation of a network by introducing the concept of label switching into an internet protocol (IP) network.
In addition, generalized multi-protocol label switching (GMPLS) is known as a technique for automatically operating distributed path networks including not only IP networks but also time division multiplexing switching (TDM) networks, such as synchronous digital hierarchy (SDH) and asynchronous optical network (SONET), and wavelength switching networks. The standardization of GMPLS is underway through discussions by Common Control and Measurement Plane Working Group (CCAMP-WG) of Internet Engineering Task Force (IETF), Optical Internetworking Forum (OIF), and International Telecommunication Union (ITU). This GMPLS has been partially put to practical use.
GMPLS realizes the standardization of the configuration of paths among different apparatuses, a bandwidth on demand (BoD) service allowing high-speed path configuration, an efficient network operation based on consolidated management of a plurality of layers, and the like.
In GMPLS, an MPLS header is given to an IP packet. This IP packet is transported through a network on the basis of the label within the MPLS header. This transporting mechanism is called label switching.
For setting paths in GMPLS, nodes cooperate with each other in the setting/releasing of paths, the setting of state information, and the like in accordance with a resource reservation protocol (RSVP). The content of the major messages exchanged using the RSVP protocol includes the following.
A Path message, propagated from upstream nodes to downstream nodes, is used to set or release paths and as a trigger for various settings.
A Resv message, propagated from downstream nodes to upstream nodes, is used to reserve a bandwidth and to respond to various settings.
A PathErr message, propagated from downstream nodes to upstream nodes, is used as an error response to a Path message.
A PathTear message, propagated from upstream nodes to downstream nodes, is used as a trigger for forced releasing of a path.
A Notify message, transmitted from a certain node to any other node, is used for notification of error information, point-to-point transmission of information, and the like.
By using these messages, the RSVP protocol allows exchanging of information among respective nodes on a point-to-point basis or using a hop-by-hop method, thereby allowing the management of paths.
FIG. 1 illustrates a sequence of steps for performing path creation (signaling) with a hop-by-hop method from a start node N1 to an end node N4, using a Path message and a Resv message of the RSVP protocol.
Referring to FIG. 1, the start node (ingress node) N1 transmits to the neighboring intermediate node N2, using a monitor line (not illustrated), a Path message including information (Explicit_Route Object: ERO) about the path from the node N1 to the node N4 and a path number (timeslot number in TDM and wavelength number in WDM) to be used between the node N1 and the node N2.
At the node N2, when a resource corresponding to the path number specified by the received Path message is not in use (valid message), the resource corresponding to the path number is entered into a reserved state. Then the node N2 transfers the same Path message to the next intermediate node N3. The node N3, by performing processing similarly to the node N2, transfers the Path message to the end node (egress node) N4.
At the node N4, when the received Path message is valid, a Resv message showing a response of “OK” is transmitted via the monitor line, and path setting at the own node (i.e., cross-connect setting of the own node) is performed.
At the node N3, which has received the Rsev message from the node N4, an Resv message is transmitted to the node N2 and path setting at the own node (i.e., cross-connect setting of the own node) is performed similarly to the above. Similar processing is performed also at the nodes N2 and N1 to complete the path setting. Note that Resv message transmission is not performed at the node N1.
Nodes on a path that have been established by using GMPLS continue to monitor the connection state of the path by periodically transmitting a Path message and a Resv message. These two messages are called Refresh messages. If a Refresh message is not received due to some reason at a certain node, this node recognizes that the corresponding path has been disconnected.
In this case at the start node N1 in GMPLS, there exists a method of partially creating a backup link in such a manner as to avoid a failed portion on the basis of information of a path disconnection notification (RFC4873: segment recovery method).
Regarding this method, only steps of creating a partial backup link have been specified. A method for detection or notification of a failure, and management of the received notification about failure information vary, depending on the implementation of respective communications apparatus manufactures.
When communication is disconnected due to generation of a failure in optical signals, loss of signal (LOS) is detected. In this case, IP packets to be transmitted/received using an optical signal of the disconnected communication are of course not transmitted/received.
In SONET, the number of bit errors per SONET frame is counted. Bit errors themselves do not directly result in the disconnection of communication, and may be often relieved by means of application level retransmission at an upper layer or an error correction mechanism. The error rate for general communication lines is specified to be 10−6 (one error per million bits) or lower. However, the error rate is actually much lower than this, and is about 10−8 (one error per 100 million bits) or lower.
Note that Japanese Laid-open Patent Publication No. 2007-36412 discloses the following technique. That is, the normality of respective links and nodes constituting a backup path is monitored. When the backup path becomes unusable, one of the two end nodes of a segment of the backup path is changed to a node on the currently used path, thereby setting a different backup path.